The present invention relates to fuel pumps for internal combustion engines, particularly those used for diesel engine fuel injection. Still more particularly, the present invention relates to a high pressure fuel pump having a variable piston area which provides variable fuel injection rates.
Fuel injectors for internal combustion engines require precisely timed delivery of pressurized fuel in order for the engine to have maximized performance and minimized harmful emissions. With respect to diesel engines, it is known that the rate at which fuel is injected affects the amount of NOx and soot emissions. Specifically, a lower rate of fuel injection during ignition delay provides a lower premixed burnt fraction, which can lower the initial formation of NOx and soot, and further lower the rate of pressure rise which translates to less combustion noise. Subsequent to the start of combustion, a higher rate of injection promotes a higher rate of diffusion combustion at lower flame temperatures. This results in lower NOx formation and higher soot oxidation. Contemporary high pressure fuel pumps provide a predetermined rate of fuel injection based upon the cam profile. As can be understood by reference to FIGS. 1 through 3, a prior art high pressure fuel pump 10 has a pump body 12 having a pump cylinder 14 formed therein. A pump piston 16 reciprocates within the pump cylinder 14, wherein a spring 18 biases the pump piston away from the head of the piston cylinder, and an external agency, such as a cam, drives reciprocation of the pump piston. At the head 14H of the pump cylinder 14 is a port 20 which communicates with a passage 22 in the pump body 12 which is interfaced with a solenoid valve 24. A fuel supply connection 26 provides fuel (at a typical pressure of 100 psig) to the solenoid valve. A high pressure fuel connection 28 is also connected with the solenoid valve 24 for supplying high pressure fuel (typically between 1,000 and 5,000 psig) to a fuel injector (or rail therefor).
In operation, a fuel pressurization chamber 30, formed in the pump cylinder 14 between the pump piston 16 and the head 14H, is filled selectively via the solenoid valve 24 with fuel from the fuel supply connection 26 when the pump piston is stroked away from the head of the pump cylinder, the fill stroke. When the pump piston is about to begin the pressurization stroke, the solenoid valve closes off the fuel supply connection and opens the high pressure fuel connection into communication with the fuel pressurization chamber. As the pump piston strokes toward the head of the pump cylinder during the pressurization stroke, the requisite fuel injection pressure is provided as high pressure fuel exits the high pressure fuel connection. In that operation of the high pressure fuel pump inevitably involves internal fuel leakage, a return fuel drain 32 is provided (filled with fuel with a typical pressurization of 10 psig), having a return fuel drain connection 34.
What remains needed in the art is a high pressure fuel pump having a variable fuel injection rate.
The present invention is a high pressure fuel pump having a variable fuel injection rate.
The high pressure fuel pump according to the present invention includes a pump body, a pump cylinder formed in the pump body and a piston reciprocable within the pump cylinder. The pump piston has a reduced diameter portion which provides a primary piston. A piston annulus is slidably and sealingly mounted on the primary piston, wherein the piston annulus provides a secondary piston. The secondary piston travel is limited by a cylinder wall abutment. The secondary piston defines a demarcation between the fuel pressurization chamber and an oppositely disposed annular actuation chamber. An actuation passage is provided between the return fuel drain and the pump cylinder at the actuation chamber (as it is defined when the primary piston is at the start of the pressurization stroke), wherein the actuation chamber passage is selectively open or closed passively by movement of the pump piston and/or dynamically by operation of an actuation solenoid valve.
In operation, if the actuation passage is open, then during the pressurization stroke the secondary piston will remain stationary relative to the pump body, in that fuel therein is able to flow out from the actuation chamber to the fuel return drain as the fuel actuation chamber contracts. On the other hand, if the actuation passage is closed, then the fuel trapped in the actuation chamber constitutes an incompressible fluid such that as the primary piston strokes toward the head of the pump cylinder during the pressurization stroke, then the secondary piston must stroke therewith in unison.
An actuation assembly provides control over movement of the secondary piston. An example of passive actuation assembly is the pump piston having a larger diameter portion than that at the reduced diameter portion of the primary piston, wherein as the pump piston strokes during the pressurization stroke, the larger diameter portion eventually occludes the entry of the actuation passage. An example of a dynamic actuation assembly is by electronic control of an actuation solenoid valve, wherein as the primary piston strokes during the pressurization stroke, the pressurization passage may be closed or open at any time for any duration by the setting of the actuation solenoid valve. The actuation assembly may be only passive, only dynamic or a combination thereof. When the secondary piston is stationary with respect to the primary piston (in other words, when the secondary piston is moving in unison with the primary piston) during the pressurization stroke, a larger amount of fuel is caused to exit the high pressure fuel connection than would be the case when the secondary piston is stationary with respect to the pump body.
Accordingly, it is an object of the present invention to provide a high pressure fuel pump having selective control over the fuel injection rate.
This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment.